Presently for the realization of measuring height differences with the topographycally necessary accuracy and in the disposable time, clear visibility is required. This has the following disadvantages:
(a) Before beginning the with leveling the felling of trees of often necessary.
(b) Aditional measurements are required around the obstacles of visibility as buildings, mountains and so on.
(c) Also it may be necessary to use spot lights or reflectors during the topographic work at night, in fog or in tunnels, mines and other places without visibility.
(d) The meterorological conditions of weather also often constrain the execution of topographical height measurements, as fog, rain and the atmospheric refraction of hot air.
(e) The same disadvantage appears in spots of great thermical refraction as foundries and furnaces.
(f) The above indicated disadvantages, aside from being very anoying, cause an increase in the operating costs and operating time.